PT'd when naked but not with undies/pullup?

I'm not sure what to do..my 2 yo is fully potty trained as long as he's naked. He catches every poo and every pee in the potty.
But as soon as you put undies on him he pees! It's like he completely forgets to pay attention if he's wearing anything.
Any ideas?

Well if he is completely potty trained with out them, let him run around comando  

Quoting Soon 2B Mom of 2:" Well if he is completely potty trained with out them, let him run around comando  "

Oh I do, at home.   I want him pt'd when we go out though so I can stop buying pull ups!
Plus when the baby comes they'll be going to my parents house for a few days and they never let him just be naked.  

I had the same problem...eventually it just clicked but it took awhile